1. Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison , Madison, Wisconsin , USA
2. Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo , Tokyo , Japan
3. The Research Center for Global Viral Diseases, National Center for Global Health and Medicine Research Institute , Tokyo , Japan
4. Pandemic Preparedness, Infection, and Advanced Research Center, University of Tokyo , Tokyo , Japan